Random delay generation for thin-film transistor based circuits

ABSTRACT

Circuits and circuit elements configured to generate a random delay, a monostable oscillator, circuits configured to broadcasting repetitive messages wireless systems, and methods for forming such circuits, devices, and systems are disclosed. The present invention advantageously provides relatively low cost delay generating circuitry based on TFT technology in wireless electronics applications, particularly in RFID applications. Such novel, technically simplified, low cost TFT-based delay generating circuitry enables novel wireless circuits, devices and systems, and methods for producing such circuits, devices and systems.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/118,184, filed Nov. 26, 2008, incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of wirelesscommunications circuits. More specifically, embodiments of the presentinvention pertain to circuits and circuit elements for generating randomdelay in wireless systems, circuits for broadcasting messages in lowfrequency (LF), high frequency (HF), very high frequency (VHF) andultra-high frequency (UHF) and radio frequency identification (RFID)systems, tags and other devices containing such circuits, systemsincluding such tags and devices, and methods for formingcollision-tolerant wireless systems.

DISCUSSION OF THE BACKGROUND

Many communication circuits require random delay generators to arbitratecollisions between competing information streams. For example, FIG. 1 isa schematic representation of an exemplary radio frequencyidentification (RFID) system. Computer 102 instructs interrogationsource 104 to generate an interrogatory signal, which is broadcast viaantenna 106 as interrogatory RF broadcast 109. RFID tags 110-1 and 110-2each receive the interrogatory RF broadcast 109, and are energizedthereby. Both tags may then attempt to broadcast a repetitive ID message(e.g., ID message 111-1 from tag 110-1 and ID message 111-2 from tag110-2) simultaneously. Antenna 106 may receive messages 111-1 and 111-2simultaneously, resulting in a collision between the ID broadcasts atdetector 108. Thus, introducing a random delay in the ID broadcast oftag 110-1, tag 110-2, or both may effectively avoid such broadcastcollisions.

FIG. 2 shows a schematic representation of an exemplary conventionalRFID tag configured to generate a repetitive ID message upon receivingan interrogatory RF signal. An interrogatory RF signal 201 is receivedby antenna 202, and the RFID tag is powered by power-up circuit 204. Anidentification message stored in memory 212 is generated and transmittedto output stage 216, with a delay introduced in the transmission of theidentification message from memory 212 to output stage 216 by a delaygenerating circuit 205 comprising clock 206, cyclic shift registers 208and 210, and delay/reset circuitry 214. The delayed identificationmessage is then transmitted by output stage 216 for broadcast by antenna202. The amount of delay introduced by delay/reset circuit 214 betweencyclic shift registers 208 and 210 may be selected so as to reduce theprobability of collision between two or more RFID tags in an RFIDsystem.

In some electronics applications, circuits and/or circuit elements maybe implemented using thin-film transistor (TFT) technology. SuchTFT-based circuits and/or circuit elements may be advantageous becauseof their low cost and broad applicability relative to conventional CMOSmanufacturing processes. For example, TFT circuitry may be implementedon a variety of substrates (e.g., on flexible substrates, such as thosecomprising or consisting essentially of organic polymers or metallicfoils) and may be fabricated using relatively economical methods (e.g.,printing). However, the complexity of circuitry which may be based onTFT technology is typically limited due to economic and technicalreasons. As a result, the number of transistors which may be availablefor implementing relatively complex circuit functions (e.g.,conventional random delay generation circuits) is somewhat limited.Since random delay generation circuits (e.g., delay generating circuit205 as shown in FIG. 2) are often critically important in minimizingcollisions in communication systems, it would be technically andeconomically advantageous to implement a simple, TFT-based random delaygeneration circuit.

SUMMARY OF THE INVENTION

In a first aspect, the present invention relates to a circuit configuredto generate a delay, including a delay element having an input terminaland an output terminal, a capacitor having a first terminal receiving aninput and a second terminal coupled to the input of the delay element,and a thin-film field-effect transistor (TFT) having a first and secondsource/drain terminals and a gate, configured to provide a currentand/or voltage to said capacitor, where the current and/or voltage has avalue that falls randomly within a predetermined range.

In a second aspect, the present invention relates to a monostableoscillator, including a capacitor, a resistive element providing acurrent and/or voltage to a first electrode of a capacitor, and afeedback path, where the current and/or voltage has a value that fallsrandomly within a predetermined range.

In a third aspect, the present invention relates to circuit configuredto broadcast a repetitive (identification) message in a wirelesscircuit, including an antenna configured to receive a power transmissionand broadcast a repetitive (identification) message, a power-up circuitproviding an initiation signal, a monostable oscillator configured toprovide a repeating timing signal, a memory element providing the(identification) message, and an output circuit configured to broadcastthe (identification) message in response to the timing signal.

In a fourth aspect, the present invention relates to an RFID systemincluding at least two RFID tags, each tag having an antenna configuredto receive a power transmission and broadcast a repetitive(identification) message, a power converting element coupled to theantenna, and a circuit configured to generate the repetitive(identification) message, where the circuit includes at least one thinfilm transistor (TFT) configured to introduce a random delay inbroadcasting the repetitive (identification) message.

In a fifth aspect, the present invention relates to a method of forminga collision tolerant wireless system, including determining a targetvariation range in the broadcast delay of a plurality of wirelessdevices having substantially the same architecture, the target variationrange being configured to reduce a broadcast collision frequency betweenthe wireless devices when the wireless devices broadcast repetitivemessages in the same read field; determining a random variation range inthe broadcast delay of the wireless devices, comparing the targetvariation range and the random variation range; and if the randomvariation range is at least equal to the target variation range, makingthe wireless devices having the random variation range in theirbroadcast delay.

The present invention advantageously provides relatively low costwireless devices with delay generating circuitry based on TFT technologyin electronics applications, particularly in RFID applications. Suchnovel, technically simplified, low cost TFT-based delay generatingcircuitry enables novel wireless circuits, devices, systems, and methodsfor producing such devices and systems. These and other advantages ofthe present invention will become readily apparent from the detaileddescription below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block representation of an exemplary conventional radiofrequency identification (RFID) system.

FIG. 2 is a block diagram of a conventional RFID tag.

FIG. 3 is a cross-sectional diagram of an exemplary TFT.

FIG. 4 is a schematic diagram of an exemplary circuit for generating arandom delay.

FIG. 5 is a schematic diagram of an exemplary monostable oscillator.

FIG. 6 is a block diagram of an exemplary wireless device forbroadcasting a repetitive message.

FIG. 7 is a flow chart embodying an exemplary method of forming acollision tolerant wireless system.

DETAILED DESCRIPTION

Reference will now be made in detail to the various embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thefollowing embodiments, it will be understood that the description is notintended to limit the invention to these embodiments. On the contrary,the invention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description, numerous specific details are set forthin order to provide a thorough understanding of the present invention.However, it will be readily apparent to one skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components, andcircuits have not been described in detail, so as not to unnecessarilyobscure aspects of the present invention.

Some portions of the detailed descriptions that follow are presented interms of processes, procedures, logic blocks, functional blocks,processing, and other symbolic representations of operations on code,data bits, data streams or waveforms within a computer, processor,controller and/or memory. These descriptions and representations aregenerally used by those skilled in the data processing arts toeffectively convey the substance of their work to others skilled in theart. A process, procedure, logic block, function, process, etc., isherein, and is generally, considered to be a self-consistent sequence ofsteps or instructions (or circuitry configured to perform or execute thesame) leading to a desired and/or expected result. The steps generallyinclude physical manipulations of physical quantities. Usually, thoughnot necessarily, these quantities take the form of electrical, magnetic,optical, or quantum signals capable of being stored, transferred,combined, compared, and otherwise manipulated in a computer or dataprocessing system. It has proven convenient at times, principally forreasons of common usage, to refer to these signals as bits, waves,waveforms, streams, values, elements, symbols, characters, terms,numbers, or the like, and to their representations in computer programsor software as code (which may be object code, source code or binarycode).

It should be borne in mind, however, that all of these and similar termsare associated with the appropriate physical quantities and/or signals,and are merely convenient labels applied to these quantities and/orsignals. Unless specifically stated otherwise and/or as is apparent fromthe following discussions, throughout the present application,discussions utilizing terms such as “processing,” “operating,”“computing,” “calculating,” “determining,” “transforming” or the like,refer to the action and processes of a computer or data processingsystem, or similar processing device (e.g., an electrical, optical, orquantum computing or processing device or circuit), that manipulatesand/or transforms data represented as physical (e.g., electronic)quantities. The terms refer to actions and processes of the processingdevices that manipulate or transform physical quantities within thecomponent(s) of a circuit, system or architecture (e.g., registers,memories, other such information storage, transmission or displaydevices, etc.) into other data similarly represented as physicalquantities within other components of the same or a different system orarchitecture.

Furthermore, in the context of this application, the terms “wire,”“wiring,” “line,” “signal,” “conductor” and “bus” refer to any knownstructure, construction, arrangement, technique, method and/or processfor physically transferring a signal from one point in a circuit toanother. Also, unless indicated otherwise from the context of its useherein, the terms “known,” “fixed,” “given,” “certain” and“predetermined” generally refer to a value, quantity, parameter,constraint, condition, state, process, procedure, method, practice, orcombination thereof that is, in theory, variable, but is typically setin advance and not varied thereafter when in use.

Similarly, for convenience and simplicity, the terms “clock,” “time,”“timing,” “rate,” “period” and “frequency” are, in general,interchangeable and may be used interchangeably herein, but aregenerally given their art-recognized meanings. Also, for convenience andsimplicity, the terms “data,” “data stream,” “bits,” “bit string,”“waveform” and “information” may be used interchangeably, as may theterms “connected to,” “coupled with,” “coupled to,” and “incommunication with” (which may refer to direct or indirect connections,couplings, or communications), but these terms are generally given theirart-recognized meanings herein. Further, a “tag” may refer to a singledevice or to a sheet and/or a spool comprising a plurality of attachedstructures, suitable for electronic article surveillance (EAS), highfrequency (HF), very high frequency (VHF), ultrahigh frequency (UHF),radio frequency (RF) and/or RF identification (RFID) purposes and/orapplications.

Traditional CMOS semiconductor fabrication methods typically employsingle crystal silicon substrates. Accordingly, in the case of a solidstate transistor, a channel region of each transistor formed in a singlecrystal silicon substrate will have predictable properties andelectrical characteristics with low variance from device to device, suchas threshold voltage, on current, or leakage current. While slightvariations in these values may occur from device to device (e.g., as aresult of doping of the substrate, processing and/or other manufacturingvariations, etc.), such variations are typically quite small due torigorous control of process variables. However, single crystal siliconwafers are relatively expensive, and rigorous control of processvariables generally requires relatively elaborate, energy intensiveprocess equipment and control mechanisms (e.g., ultrahigh vacuumchambers, process equipment for handling toxic, corrosive, and/orpyrophoric gaseous feedstocks, cleanrooms and rigorous cleanroomprotocols, etc.), which result in higher costs typically associated withtraditional CMOS semiconductor fabrication methods.

There is generally a much greater device-to-device variabilityassociated with the fabrication of thin-film transistors. TFTs aregenerally not manufactured on single crystal silicon under therigorously controlled conditions associated with traditionalsemiconductor fabrication methods. Consequently, there is generallyincreased variability associated with TFT fabrication processes relativeto conventional semiconductor fabrication processes. FIG. 3 shows ageneric representation of a TFT 300. TFT 300 comprises a semiconductorsubstrate or body 302 deposited on a substrate 301. Undoped and/or dopedsemiconductor precursor inks comprising undoped and/or dopedpolysilanes, heterocyclosilanes and/or undoped and/or dopedsemiconductor nanoparticles may be deposited or printed by a suitableprinting method (e.g., inkjet, offset lithography, screen printing,etc.) in a desired pattern on substrate 301, then cured and/or annealedto form semiconductor body 302. Semiconductor precursor inks comprisingpolysilanes may be described in U.S. Pat. Nos. 7,422,708, 7,553,545,7,498,015, and 7,485,691, and U.S. patent application Ser. No.11/867,587, filed Oct. 4, 2007, the relevant portions of each of whichare incorporated herein by reference. Semiconductor precursor inkscomprising heterocyclosilanes may be described in U.S. Pat. No.7,314,513, U.S. patent application Ser. No. 10/950,373, filed Sep. 24,2004 and U.S. patent application Ser. No. 10/956,714, filed Oct. 1,2004, the relevant portions of each of which are incorporated herein byreference. Semiconductor precursor inks comprising doped polysilanes maybe described in U.S. patent application Ser. No. 11/867,587, filed Oct.4, 2007; the relevant portions of each of which are incorporated hereinby reference. Semiconductor precursor inks comprising undoped and/ordoped semiconductor nanoparticles may be described in U.S. patentapplication Ser. No. 10/616,147, filed Jul. 8, 2003 the relevantportions of which are incorporated herein by reference. Alternatively,the semiconductor body 302 can be formed by one or more conventionalthin-film processes or techniques.

A gate dielectric 304 may also be formed via a printing process (e.g.,printing and/or deposition of a dielectric layer, etc.; see, e.g., U.S.Prov. Pat. Appl. No. 61/118,419, filed Nov. 26, 2008; the relevantportions of which are incorporated herein by reference) or by aconventional method (e.g., via deposition of an oxide or oxidation of asurface of a semiconductor film to form an oxide film, then patterningof the oxide film, etc.). Gate electrode 305 may similarly be formed bya printing process or by a conventional method, and may be formed from ametal or a semiconducting material. Source and drain regions 303 mayalso be formed by any number of methods (e.g., printing or depositing adopant layer on semiconductor body 302, followed by a drive-in step; ionimplantation, etc.; see, e.g., U.S. patent application Ser. No.11/888,942, filed Aug. 3, 2007, and U.S. patent application Ser. No.11/888,949, filed Aug. 3, 2007; the relevant portions of each of whichare incorporated herein by reference). Furthermore, metal lines orinterconnects connecting one or more additional devices and/or circuitelements to TFT 300 may also be formed via printing processes (see,e.g., U.S. patent application Ser. No. 12/175,450, filed Jul. 17, 2008and U.S. patent application Ser. No. 12/131,002, filed May 30, 2008; therelevant portions of each of which are incorporated herein byreference), or by conventional thin-film or blanketdeposition/photolithography processes.

Representative TFT 300 has been shown with regular, well definedfeatures and regular dimensions. However, each of the fabricating stepsin a method for making TFT 300 may have some variability associatedtherewith. Methods for making TFT 300 (or similar printed devices and/orcircuit elements which may exhibit similar variability) may includeprinting one or more elements of TFT 300 (see, e.g., U.S. patentapplication Ser. No. 11/452,108, filed Jun. 12, 2006, U.S. patentapplication Ser. No. 11/888,942, filed Aug. 3, 2007, U.S. patentapplication Ser. No. 11/888,949, filed Aug. 3, 2007, U.S. patentapplication Ser. No. 11/818,078, filed Jun. 12, 2007, U.S. patentapplication Ser. No. 11/203,563, filed Aug. 11, 2005, and U.S. patentapplication Ser. No. 12/243,880, filed Oct. 1, 2008; the relevantportions of each of which are incorporated herein by reference).Accordingly, the shape of each of the features of TFT 300 may vary amongmanufacturing runs and/or from device to device.

For example, semiconductor body 302 may have a height H that varieswithin a known range H±x, where H is a target value for the height ofsemiconductor body 302 and x corresponds to the variability associatedwith the process for manufacturing TFT 300. Thus, in a production run ofTFT 300, semiconductor body 302 may have a height ranging from (H−x) to(H+x). Similarly, the channel region in TFT 300 may have a length L thatvaries within a known range L±y, where L is a target value for thelength of semiconductor body 302 and y corresponds to the variabilityassociated with the process for manufacturing TFT 300. Thus, a channelregion of TFT 300 may have a length ranging from (L−y) to (L+y).Further, a channel region of TFT 300 may have a width W that may varieswithin a known range W±z, where W is a target value for the width ofsemiconductor body 302 and z corresponds to the variability associatedwith the process for manufacturing TFT 300. Thus, the width of a channelregion of TFT 300 may range from (W−z) to (W+z). Thus, in a productionrun of TFT 300, the dimensions of the channel region from device todevice may vary, for example, within the ranges of H±x, L±y and W±z asdescribed above. Furthermore, while TFT 300 is shown with elementshaving rectilinear shapes and/or dimensions, the features of TFT 300 mayhave irregular shapes. For example, semiconductor body 302 and/or otherfeatures of TFT 300 may have a dome-shaped profile (see, e.g., U.S.patent application Ser. No. 12/243,880, filed Oct. 1, 2008; the relevantportions which are incorporated herein by reference), and the dimensionsof such features may vary across the shape/and or profile of thefeatures within ranges as described in reference to the exemplaryrectilinear structure of semiconductor body 302.

Similarly, the formation of every other feature of TFT 300 (e.g.,source/drain regions 303, gate dielectric 304, and gate electrode 305)will have a number of process and/or material variables associatedtherewith (e.g., size, shape, thickness, composition, etc.) that mayvary within a range associated with the process for manufacturing thefeatures of TFT 300. For example, in the case of printed films orfeatures, an ink may exhibit variations in the concentration of inkcomponents over the course of a print run, and printed films or featuresmay accordingly have (slightly) different chemical compositions. Inaddition, in any process step, undesired contaminants may be introduced,possibly resulting in additional variations between devices.Furthermore, each process variable associated with a particular printingor deposition method may affect the morphology of an individual feature,and consequently may increase (within determinable tolerances) thevariation between printed features.

A printed or deposited amorphous semiconductor film or feature may besubsequently crystallized to form a polycrystalline semiconductor film,to improve the electrical characteristics of the semiconductor material.However, the process for crystallizing an amorphous semiconductor filmor feature (e.g., a thermal process, laser process, etc.) may producecrystalline regions having somewhat variable crystal structures,orientations and/or crystallinity percentages and/or proportions, andthe regions themselves may vary within known tolerances in size andshape. These variations in polycrystalline films or features formed froma printed semiconductor precursor ink may also contribute to variationsbetween devices incorporating such polycrystalline films or features.Furthermore, typical substrates for use in TFTs include low cost glass,metal foil and/or polymer substrates, which have properties that vary(e.g., surface uniformity) more than those of a single crystalsubstrate. Variations in the surface characteristics of the substrate atdifferent locations on the substrate (e.g., roughness, wettability,surface energy, etc.) may introduce additional variations betweendevices.

Consequently, there is typically some variation in the structure and/orperformance of TFTs from device to device. Typical features of a TFTthat may vary include film or feature composition (e.g., chemicalcomposition, impurities); length, width and two-dimensional shape; filmthickness; surface characteristics of a film or feature; crystallineregion size, orientation and distribution in a channel film; etc. Thetotal variation in the printed TFTs across a manufacturing lot of thedevices is typically a combination of minor variations associated withthe printing process and/or materials used for constructing the printedTFTs, and may vary over a statistically predictable or predeterminedrange.

Structural variations in a TFT result in concomitant variations in theelectrical characteristics of a TFT. For example, electricalcharacteristics such as leakage current, on-current, or the thresholdvoltage of the TFT may vary as a result of variations in the structureand/or composition of a TFT. As previously described with respect to TFT300, a channel region of TFT 300 may have a length, width and heightthat vary within the tolerances of a manufacturing process. Accordingly,the electrical characteristics of TFT 300, such as channel resistance,will vary as a function of the channel dimensions, composition, etc.Accordingly, the electrical characteristics of a TFT may vary betweenTFTs produced by a given manufacturing process within the tolerances ofsuch a manufacturing process.

Due to the above-described device-to-device variations typicallyassociated with TFTs, they are typically used in devices, circuitsand/or applications that are either tolerant to such random variationsin the electrical characteristics of the TFT, or include additionalelements, arrangements, connectivity, etc. that may compensate for suchvariations. However, the nature of the variations in TFT manufacture mayfortuitously provide a novel mechanism for introducing a random delayinto a system, where such a system may employ a random delay element.

The present invention advantageously employs random variations in theelectrical characteristics of TFTs associated with a manufacturingprocess as described above to provide a simple, low-cost means ofimplementing a random delay in circuits and/or systems such as RFIDsystems where random delays in the transmission of multipleidentification messages from multiple RFID tags may prevent collisionsbetween such messages.

The invention, in its various aspects, will be explained in greaterdetail below with regard to exemplary embodiments.

An Exemplary Circuit Configured to Generate a Delay

In a first aspect, the present invention relates to a circuit configuredto generate a delay, including a delay element having an input terminaland an output terminal, a capacitor having a first terminal receiving aninput and a second terminal coupled to the input of the delay element,and a thin-film field-effect transistor (TFT) having a first and secondsource/drain terminals and a gate, configured to provide a currentand/or voltage to said capacitor, where the current and/or voltage has avalue that falls randomly within a predetermined range.

The TFT may comprise one or more semiconductor layers (e.g., atransistor channel layer, a source/drain terminal layer, and/or one ormore intrinsic and/or lightly- or heavily-doped diode layers); a gateinsulator layer on or over at least one of the semiconductor layers; agate metal layer on the gate insulator layer; a plurality of metalconductors in electrical communication with the gate metal layer and thesource and drain terminals; and one or more dielectric layers betweenvarious metal conductors and/or semiconductor layer(s). Exemplarysemiconductor, dielectric and metal layers of a TFT as described herein,and materials and methods for forming such a TFT are described ingreater detail in U.S. Pat. No. 7,619,248 and U.S. patent applicationSer. Nos. 11/203,563, 11/243,460, 11/452,108, 11/888,949, 11/888,942,11/818,078, 11/842,884, 12/175,450, 12/114,741, 12/131,002 and12/243,880, filed on Aug. 11, 2005, Oct. 3, 2005, Jun. 12, 2006, Aug. 3,2007, Aug. 3, 2007, Jun. 12, 2007, Aug. 21, 2007, Jul. 17, 2008, May 2,2008, May 30, 2008, Oct. 1, 2008, the relevant portions of each of whichare incorporated herein by reference.

In some embodiments, the TFT may be a printed TFT. Forming the printedTFT generally includes printing at least one layer comprising asemiconducting material in a first pattern on a substrate. Printing thelayer(s) of the TFT may comprise printing an ink that includes one ormore semiconductor (e.g., silicon) precursor(s), metal precursors, ordielectric precursors in a solvent in which the semiconductor, metal, ordielectric precursor(s) are soluble. For example, the semiconductorprecursor may comprise silicon nanoparticles and/or an oligo- and/orpolysilane, which may be doped or undoped. For further details, see U.S.Pat. Nos. 7,314,513 and 7,485,691 and U.S. patent application Ser. No.11/867,587 filed on Oct. 4, 2007, the relevant portions of each of whichare incorporated herein by reference.

In various embodiments, forming the TFT on a substrate further comprisesprinting a second layer of a second material in a second pattern on orabove the first pattern. The second material may comprise a dielectricprecursor, such as a molecular, organometallic, polymeric and/ornanoparticle precursor in a solvent or solvent mixture in which thedielectric precursor is soluble. In some embodiments, the dielectricprecursor is a source of silica, silicon nitride, silicon oxynitride,aluminate, titanate, titanosilicate, zirconia, hafnia, or ceria.Preferably, the solvent or solvent mixture for such embodimentscomprises a high volatility solvent in an amount of at least 10 wt %relative to a second solvent or solvent mixture, and a low volatilitysolvent in an amount of at least 10 wt % relative to a second solvent orsolvent mixture. In other embodiments, the dielectric precursor is anorganic polymer or precursor thereof (e.g., an acrylic polymer), and thesolvent or solvent mixture comprises a relatively high-viscosity, lowvolatility solvent or solvent mixture. The second material may furthercomprise a dopant precursor containing a dopant element selected fromthe group consisting of boron, phosphorous, arsenic, and antimony.

In a further embodiment, forming the TFT on the substrate furthercomprises printing a third layer of a third material in a third patternon or above the first and/or second pattern(s). The third material maycomprise a metal precursor, in which the metal precursor comprises oneor more Group 4, 5, 6, 7, 8, 9, 10, 11, or 12 metal salts, complexes,clusters and/or nanoparticles in a third solvent or solvent mixtureadapted to facilitate coating and/or printing of the metal precursor. Incertain embodiments, the metal precursor comprises a metal salt,compound and/or complex having one or more ligands bound thereto thatform gaseous or volatile byproducts upon reduction of the metal salt,compound and/or complex to an elemental metal and/or alloy thereof. Thethird material may further comprise one or more additives (e.g., one ormore reducing agents) that can also form gaseous or volatile byproductsupon reduction of the metal salt, compound and/or complex to anelemental metal and/or alloy. Such metal formulations enable theprinting of a pure metal film using metal precursor(s) and reducingagent(s) that generally do not leave substantially adverse levels ofimpurities and/or residues in the film. For additional details, see U.S.patent application Ser. No. 12/131,002, filed May 30, 2008, the relevantportions of which are incorporated herein by reference.

For each printed layer of the TFT, the printed precursor ink(s) isgenerally dried and cured. The lengths of time and the temperatures atwhich the ink is dried and the dried precursor(s) are cured depend uponthe particular ink formulation and the particular precursor(s), but ingeneral, the ink is dried at a temperature and for a length of timesufficient to remove substantially all of the solvent from the printedink, and the dried precursor(s) are cured at a temperature and for alength of time sufficient to convert the precursor(s) to the material ofthe final film (e.g., a semiconductor, dielectric or metal). Additionaldescription of exemplary printed TFTs and methods of making such printedTFTs are described in U.S. patent application Ser. No. 11/805,620, filedMay 23, 2007 and U.S. patent application Ser. No. 12/243,880, filed Oct.1, 2008, the relevant portions of each of which are incorporated hereinby reference.

The present TFT may be formed by employing an “all printed” process, bya process employing a mixture of printing and conventional processingsteps, or by conventional processing methods. The TFT may be an NMOStransistor or a PMOS transistor, and may be electrically connectedand/or configured to function as, e.g., a transistor, a diode, aresistor, a capacitor, an off-connected TFT and/or any other possibleconfiguration of the TFT compatible with the present circuit. In thepresent circuit, the TFT is configured to provide a current and/orvoltage to one terminal of a capacitor. As described above, the TFT haselectrical characteristics associated therewith such as a leakagecurrent, on-current, or threshold voltage that will vary within thetolerances of the manufacturing process utilized in making the TFT. Forexample, the random value(s) of the electrical parameters such as aleakage current, on-current, or threshold voltage may be associated withvariability in the dimensions, shape, topology and/or composition of theTFT features, layers and/or components (e.g., body, gate, source/drainregions, etc.).

In certain embodiments, the TFT may comprise a material that provides apredetermined range of leakage current, on-current, or threshold voltagevalues. Such a material may be employed in forming a channel region,source/drain region(s), or a gate electrode. For example, the TFT maycomprise one or more semiconductor layer(s) that function as a channelregion, source/drain region(s), or a gate electrode. In someembodiments, the same material may be used to form a channel region andsource/drain region(s). The size and/or shape of the semiconductor layerthat functions as the channel region, source/drain region(s), or a gateelectrode of the TFT may vary within certain known limits (e.g., withinthe variability range associated with the method for depositing thechannel material as described herein). Thus, the characteristics of thematerial that comprises the channel region, source/drain region(s), orgate electrode, such as the composition, length, width, thickness,topography, crystalline structure, or other characteristics, will affectthe value of the electrical characteristics of the TFT such as leakagecurrent, on-current, or threshold voltage. For example, a longer channelin a TFT generally results in a higher resistance associated withcurrent flow from source to drain, thus affecting the value of theon-current and threshold voltage of the TFT comprising such a longerchannel relative to a TFT having a shorter channel.

In some embodiments, the material that provides the predetermined rangeof a leakage current, an on-current, or threshold voltage of the TFT maybe amorphous silicon or polysilicon, which may be doped with a dopant,such as an n-type dopant or a p-type dopant (e.g., B, P, As, Sb, etc.).In certain embodiments, the amorphous or polycrystalline silicon isdoped with an amount of dopant sufficient to control a threshold voltageof a TFT containing such doped amorphous or polycrystalline silicon. Forexample, the amorphous or polycrystalline silicon may have a dopantconcentration of from about 10¹⁶ to about 5×10¹⁸ atoms/cm³. In addition,the doped amorphous or polysilicon material forming a channel region ofa TFT may have a geometry (e.g., length, width, or thickness asdescribed above) that determines the predetermined range of current orvoltage that the TFT may provide to downstream circuit elements.Accordingly, the manufacturing parameters that determine the geometry ofthe features of the TFT of the present circuit may be varied to obtain adesired target range of electrical parameters associated with the TFT.For example, a shorter channel and/or a lower dopant concentration in achannel region of a TFT may lower the threshold voltage of the TFT.Conversely, a longer channel region may afford a higher thresholdvoltage.

The present circuit configured to generate a delay further includes acapacitor having a first terminal receiving an input, and a secondterminal coupled to an input of a delay element and a source/drainterminal of the TFT, so as to receive a current and/or voltage from theTFT. Generally, capacitor comprises a first undoped or dopedsemiconducting layer and a second undoped or doped semiconducting layer,with a dielectric layer therebetween electrically isolating the twosemiconducting layers. In some embodiments, however, the electricallyactive layers of the capacitor may comprise one or more metals and/oralloys. The capacitor may also include contacts in electricalcommunication with the first and second undoped or doped semiconductinglayers, configured to electrically connect the capacitor to one or moreof a TFT, a delay element and/or other circuit elements. The contactsgenerally comprise a metal, and may be formed by one of the printingtechniques described above. Thus, for example, printing the contactsgenerally comprises printing a first metal precursor ink in a pattern,optionally forming a first undoped or doped semiconducting layer,forming the intervening capacitor dielectric layer, optionally forming asecond undoped or doped semiconducting layer, and then printing a secondmetal precursor ink in a pattern on the second semiconducting layer, orplating a second metal on a patterned on a patterned semiconductorand/or metal layer.

The present capacitor may be produced according to manufacturingprocesses used to make the TFT, thus enabling construction of both theTFT and the capacitor in at least some concurrent steps. For example,the capacitor may be made by depositing layers of undoped or dopedsemiconducting material, dielectric material and/or metal on asubstrate. In some embodiments, the capacitor may be formed by employingan “all printed” process, by a process employing a mixture of printingand conventional processing steps, or by conventional processingmethods. Exemplary capacitors and methods for making such capacitors aredescribed in U.S. patent application Ser. No. 11/452,108, filed Jun. 12,2006, U.S. patent application Ser. No. 12/249,841, filed Oct. 10, 2008and U.S. patent application Ser. No. 12/243,880, filed Oct. 1, 2008, therelevant potions of each of which are incorporated herein by reference.The capacitor may have a capacitance value selected in conjunction witha resistance value of the TFT to provide a desired RC time constant τ(e.g., associated with a resistance R of the TFT and a capacitance C ofthe capacitor), as will be described in detail below in reference to theexemplary circuit 400 of FIG. 4.

The present circuit further comprises a delay element having an inputterminal and an output terminal, configured to receive an input signaland provide an output signal. The delay element input terminal may beelectrically connected to a source/drain terminal of the TFT, and oneterminal of the capacitor of the present circuit. The delay elementgenerally receives an input signal, and delays the signal for apredetermined period of time before providing an output signal. However,the delay element may also function in concert with the present TFT andcapacitor to effect the delay of a signal. The delay element of thepresent circuit may comprise one or more logic gates configured to delayan input signal for a predetermined period of time. The logic gate(s)may be selected and/or configured to provide a desired delay and/oroutput in response to an input. In some embodiments, the delay elementcomprises a plurality of inverting logic gates having an input coupledto (i) the capacitor of the present circuit and (ii) an output of theTFT. Alternatively, the delay element may comprise or consistessentially of an inverter. Delay elements comprising logic gates asdescribed herein may be designed and implemented according to procedureswell known to those skilled in the art.

For example, the delay element may comprise one or more logic gates(e.g., inverters, etc.), which may be constructed using ComplementaryMetal Oxide Semicondonductor (CMOS) logic. Thus, the logic gate(s) inthe present delay element may comprise one or more TFTs. Accordingly,TFTs in such logic gates may be produced according to manufacturingprocesses used to make the TFT providing a current and/or voltage to thecapacitor, advantageously enabling construction of the TFT, thecapacitor, and the delay element in at least some concurrent steps. Forexample, TFTs in a delay element of the present circuit may be made bydepositing layers of semiconducting material (which may or may notcontain a dopant), dielectric material and/or metal on a substrate asdescribed above. Accordingly, the elements of the present circuit forgenerating a delay may be formed simultaneously by employing an “allprinted” process or by a process employing a mixture of printing andconventional processing steps to form the present circuit.

FIG. 4 shows an exemplary embodiment of a circuit configured to generatea delay, comprising an NMOS off-connected TFT 402, a capacitor 401, andan inverter 403. TFT 402 has a first source/drain terminal having aninput signal V_(DD). V_(DD) is generally a DC supply voltage, and may beessentially any value compatible with delay circuit 400, typically onthe order of 5 V or less, or any other voltage or range of voltagescompatible with digital logic circuits and/or circuit elements. Inexemplary circuit 400, TFT 402 is shown with a second source/drainterminal electrically connected to the gate electrode of TFT 402 in anoff-connected configuration. The second source/drain terminal of TFT 402is electrically connected to a second terminal of capacitor 401 and aninput of inverter 403. Capacitor 401 receives an input V_(IN) on a firstterminal, and has a second terminal connected to an input of inverter403. Inverter 403 is configured to receive a voltage V_(X) at an inputterminal, and provide an output V_(OUT). Additional inverters may belinked in series with inverter 403, as desired. In some embodiments,V_(OUT) may be a timing signal (e.g., the delay signal) provided to adownstream circuit and/or circuit element (see, e.g., the discussion ofFIG. 5 below).

The function(s) of exemplary delay circuit 400 will now be explainedwith reference to FIG. 4. At a time prior to inputting a signal V_(IN),delay circuit 400 is powered up, V_(IN) is high, both plates ofcapacitor 401 are at a voltage V_(DD) (e.g., the DC source voltage forcircuit 400), and output V_(OUT) is low. At a time T=0, an input signal(e.g., V_(IN)→0) for a short duration (e.g., less than a period of timeτ) is provided to the first terminal of capacitor 401. Capacitor 401 ispulled low since it is discharged, and V_(OUT) switches to a high state(e.g., V_(DD)). Capacitor 401 then charges via a leakage current throughTFT 402. The flow of current through TFT 402 charges capacitor 401, andat a time T=τ (e.g., the RC time constant τ of the circuit) whencapacitor 401 is charged, inverter 403 changes the output state andV_(Out)→0. Thus, the leading edge of input signal V_(IN) is delayed by aduration τ at the output V_(OUT). The duration of time t varies fromdevice to device in accordance with random variations in the capacitanceof capacitor 401 and the resistance of TFT 402, since τ is a directfunction of both parameters.

As described above, TFT 402 provides a current in circuit 400 that is adirect function of the overall resistance of TFT 402. Furthermore, theelectrical characteristics of TFT 401, including the overall resistanceof TFT 402, may vary randomly within the tolerances associated with amanufacturing process by which TFT 402 is produced. Thus, since time τis a direct function of the on-current (via the resistance) of TFT 402,the duration of the delay time τ will have a random value that is afunction of the tolerances associated with a manufacturing process bywhich TFT 402 is produced. For example, since TFT 402 can be configuredin an off-connected mode, the gate can always be at a voltage lower thanthe threshold voltage of the TFT. In an alternative, the gate of TFT 402can be connected to ground potential (e.g., V=0). Thus, current throughTFT 402 may be a leakage current only, which may vary by as much asseveral orders of magnitude due to variations between TFTs, andaccordingly, τ can vary greatly from device to device.

An Exemplary Monostable Oscillator

In another aspect, the present invention concerns a monostableoscillator including a capacitor, a resistive element providing acurrent and/or voltage to a first electrode of the capacitor, and afeedback path, where the current and/or voltage has a value that fallsrandomly within a predetermined range.

In some embodiments of the present monostable oscillator, the resistiveelement comprises a TFT. The TFT may be as described above, and maycomprise, for example, an NMOS TFT or a PMOS TFT. The TFT may beelectrically connected and/or configured to function as, e.g., atransistor, a diode, a resistor, a capacitor, an off-connected TFTand/or any other possible configuration of the TFT that provides aresistive element. In certain embodiments, the resistive element is anoff-connected TFT (e.g., with the gate and a source/drain terminalelectrically connected and held to a voltage that keeps the TFT off).The resistive element is not limited to a TFT, and may be a resistor, adiode, or any other circuit element or combination of elements thatprovides a variable resistance to the flow of current from device todevice.

In the present monostable oscillator, the resistive element isconfigured to provide a current and/or voltage at one terminal of acapacitor. Generally, the predetermined range within which the currentand/or voltage supplied by a TFT resistive element of the presentmonostable oscillator depends upon the operating range of the TFT. Asdescribed above, a TFT resistive element has electrical characteristicsassociated therewith such as a leakage current, on-current, and/orthreshold voltage that vary within the tolerances of the manufacturingprocess utilized in making the TFT. Thus, the randomness of the currentand/or voltage provided by a TFT resistive element will vary as afunction of the variations associated with the manufacturing process formaking the TFT. In some embodiments, a TFT resistive element comprises amaterial that provides a predetermined range of leakage current,on-current, and/or threshold voltage, as previously described herein.The present monostable oscillator further includes a capacitor aspreviously described above, and may be produced according to themanufacturing processes described elsewhere herein.

The present monostable oscillator further comprises a feedback path,which may comprise a printed semiconducting material or metal (see,e.g., U.S. patent application Ser. No. 12/175,450, filed Jul. 17, 2008and U.S. patent application Ser. No. 12/131,002, filed May 30, 2008; therelevant portions of each of which are incorporated herein byreference). The feedback path may electrically connect a terminal of theresistive element, a first terminal of the capacitor, and a secondterminal of the capacitor. There may be additional circuit elements(e.g., logic gates) interposed in the feedback path. Thus, in certainembodiments, the feedback path comprises one or more logic gates. Thelogic gate(s) may be configured to provide a desired delay and/or outputin response to an input. In some embodiments, the logic gate(s) includeinverter logic having an input coupled to a terminal of the resistiveelement and a terminal of the capacitor, and having an output comprisinga timing signal (e.g., the delay signal). Logic gates as describedherein may be designed and implemented according to procedures wellknown to those skilled in the art.

FIG. 5 shows an exemplary monostable oscillator 500, comprising a TFTresistive element 504, a capacitor 502, an inverter 503, a feedback path505, and a XNOR gate 501. TFT 504 has a first source/drain terminalhaving an input signal V_(DD). V_(DD) is generally a DC supply voltage,and may be essentially any value compatible with monostable oscillator500, typically on the order of 5 V or less, or any other voltage orrange of voltages compatible with digital logic circuits and/orthin-film circuit elements. In exemplary monostable oscillator 500, TFT504 is an NMOS TFT with a source/drain terminal electrically connectedto the gate electrode of TFT 504 in an off-connected configuration. Asecond source/drain terminal of TFT 504 is electrically connected to asecond terminal of capacitor 502 and an input inverter 503. Capacitor502 receives an input V_(A) at node A from XNOR gate 501, and has asecond electrode connected to an input of inverter 503 at node B.Inverter 503 is configured to receive a voltage V_(B) at an inputterminal, and provides an output V_(OUT). Additional inverters may belinked in series with inverter 503, as desired. In some embodiments,V_(OUT) may be a timing signal (e.g., the delay signal) provided to adownstream circuit and/or circuit element (e.g., a counter that controlsthe output of identification and/or other data from the device/tag) andfed back to XNOR gate 501 along feedback path 505. XNOR gate 501 isconfigured to receive two inputs, V_(IN) (e.g., a trigger or initiationpulse), and V_(OUT) from feedback path 505.

The function(s) of exemplary delay circuit 500 will now be explainedwith reference to FIG. 5. At a time prior to inputting a signal V_(IN),monostable oscillator 500 is powered up, providing a voltage V_(DD)(e.g., the DC source voltage for circuit 500) to TFT 504. Trigger signalV_(IN) is low, both plates of capacitor 502 (and nodes A and B) are atV_(DD), and output V_(OUT) is low. At time T=0, an input signal (e.g.V_(IN)→V_(DD)) is provided to XNOR gate 501 for a short duration (e.g.,less than a period of time τ), pulling down the voltage on capacitor 502(e.g., V_(A)→0 and V_(B)→0), and switching the output of inverter 503 toa high state (e.g., V_(OUT)→V_(DD)). Capacitor 501 then charges via aleakage current through TFT 504. The flow of current through TFT 504charges capacitor 502, and at a time T=τ (e.g., the RC time constant τof the circuit) when capacitor 502 is charged (e.g., V_(A)=V_(DD) andV_(B)=V_(DD)), inverter 503 changes the output state and V_(OUT)→0.Thus, the leading edge of input signal V_(IN) is delayed by a duration τat the output V_(OUT). The duration of time τ can be varied inaccordance with random variations in the capacitance of capacitor 502and the resistance of TFT 504, since τ is a direct function of bothparameters. The present monostable oscillator is retriggerable, and thecycle can be triggered again by repeating input V_(IN) at a desiredfrequency. The duration of trigger signal V_(IN) is not particularlylimited, and may be any duration less than τ.

In addition, other logic can be used that is functionally equivalent toXNOR gate 501, alone or in combination with inverter 503. Also, inverter503 may comprise a plurality of serially-connected inverters (e.g., 2ninverters or 2n+1 inverters) depending on the logic gate(s) receivingV_(IN) and V_(OUT) from feedback path 505. Thus, a desired input and/oroutput may be effected by employing one or more logic gate(s) asdescribed herein.

As described above, TFT 504 provides a current in exemplary monostableoscillator 500 that is a direct function of the overall resistance ofTFT 501. Furthermore, the electrical characteristics of TFT 504,including the overall resistance of TFT 504 may vary randomly within thetolerances associated with a manufacturing process by which TFT 504 isproduced. Thus, since time t is a direct function of the on-current (viathe resistance) of TFT 504, the duration of the delay time t will have arandom value that is a function of the tolerances associated with amanufacturing process by which TFT 501 is produced.

An Exemplary Circuit Configured to Broadcast a Repetitive Message

A further aspect of the invention relates to a circuit configured tobroadcast a repetitive message in a wireless system including an antennaconfigured to receive a power transmission and broadcast a repetitiveidentification message, a power-up circuit providing an initiationsignal, a monostable oscillator configured to provide a repeating timingsignal, a memory element providing the identification message, and anoutput circuit configured to broadcast the identification message inresponse to the timing signal. In some embodiments, the circuit includesone or more shift register(s) and a clock generator configured toprovide a clock signal to the one or more shift register(s). Exemplarycircuits and methods for making such circuits are described in U.S.patent application Ser. No. 11/452,108, filed Jun. 12, 2006, U.S. patentapplication Ser. No. 11/544,366, filed Oct. 6, 2006, U.S. patentapplication Ser. No. 11/203,563, filed Aug. 11, 2005, and U.S. patentapplication Ser. No. 12/249,707, filed Oct. 10, 2008, the relevantportions of each of which are incorporated herein by reference.

In some embodiments, the present circuit is configured to broadcast arepetitive identification message in an RFID system. For example, FIG. 6is a block diagram of an exemplary circuit 600 configured to broadcast arepetitive identification message in an RFID system, including antenna603, power-up circuit 604, clock subcircuit 605, cyclic shift registers606 and 607, memory portion 608, random delay circuit 609 includingmonostable oscillator 611 and output circuit 610. Antenna 603 may beimplemented using a resonant LC circuit for use at 13.56 MHz, forexample. Alternatively, the antenna may be implemented using a dipole orsimilar such antenna for 900 MHz or 2.4 GHz operation. However, thepresent circuit may employ antennas that operate in the LF, HF, VHF, andUHF regimes (e.g. 100-150 KHz, 5-15 MHz, 800-1000 MHz, and 2.4-2.5 GHz).Such devices are described in further detail in U.S. patent applicationSer. Nos. 11/452,108 and 12/467,121 filed Jun. 12, 2006 and May 15,2009, the relevant portions of each of which are incorporated herein byreference.

Generally, the antenna may be used to provide power for operation of thetag circuitry, and to provide information from the tag to a tag readeror interrogator (e.g., a repetitive identification message). Usingpower-up circuit 604, power can be extracted by rectifying an RF signalcollected by antenna 603 and storing the resultant charge in a storagecapacitor. Thus, when a tag enters a region of space with sufficientelectromagnetic radiation being transmitted from a nearby reader, thestorage capacitor begins to charge-up, and a voltage across thecapacitor increases accordingly. When the voltage reaches a sufficientvalue, an “enable” signal can be generated, and this enable signal canbe used to initiate the operation of circuit 600 (e.g., by powering upand initiating the respective functions of clock 605, cyclic shiftregisters 606 and 607, and random delay circuit 609 including monostableoscillator 611).

In an exemplary clocking subcircuit (e.g., 605), a clock signal can begenerated so as to synchronously operate associated circuitry (e.g.,cyclic shift registers 606 and 607). This clock signal may be generatedby dividing down the incident RF signal received by antenna 603, bygenerating a local clock signal using an on-chip oscillator, or bydemodulating a reader-provided clock signal from the received RF signal.This clock signal may be used to drive cyclic shift register 606, whichmay begin shifting a single predetermined state (e.g., a binary “high”bit) through all the rows addressing the memory, thus selecting one rowof memory at a time. The output of cyclic shift register 606 may in turnbe used to clock a second cyclic shift register 607, thus shifting asingle high bit through all the columns addressing the memory, thusselecting a single column of memory at a time.

Random delay circuit 609 may comprise a monostable oscillator 611, whichmay include a capacitor, a resistive element providing a current and/orvoltage to a first electrode of the capacitor, and a feedback path,where the current and/or voltage has a value that falls randomly withina predetermined range (e.g., such as the exemplary monostable oscillatorof FIG. 5). As described above, a resistive element in monostableoscillator 611 provides a current that is a direct function of theoverall resistance of the resistance element. The electricalcharacteristics of the resistance element may vary randomly within thetolerances associated with a manufacturing process by which theresistance element is produced is produced. Thus, since the duration τof the delay of a signal through monostable oscillator 611 is a directfunction of current [τ=f(RC) as previously described herein] through adelay element in monostable oscillator 611, the duration of the delaytime τ will have a random value that is a function of the RC timeconstant of, e.g., a (printed) TFT delay element and a (printed)capacitor in monostable oscillator 611. The random element in the RCtime constant generally is determined by the tolerances associated witha manufacturing process by the delay element in which monostableoscillator 611 is produced. Furthermore, monostable oscillator 611 canreceive a repeating initiation signal and/or provide a repeating delayedoutput accordingly.

In operation, as cyclic shift registers 606 and 607 go through theirsequence, various bits or portions of a predefined bit string can betransferred back to the reader. At the end of the sequence, the randomdelay circuit 609 can be triggered by the output of cyclic shiftregister 607 to cause circuit 600 to “go silent” and remain in thissilent state for an interval determined by random delay circuit 609. Aspreviously described, the delay period will have a duration that fallsrandomly within a predetermined range determined by monostableoscillator 611. The delay period may also be affected by, e.g.,environmental or physical parameters such as temperature, powerdelivered to the tag, and/or electrical performance of variouscomponents within the delay circuit. When random delay circuit 609completes its cycle, it can reset shift registers 606 and 607, and theoverall process can be repeated.

Bits provided from memory 608 in circuit 600 may be passed to outputstage 610, and transmitted via antenna 603 for information (e.g., in theform of a bit string) transfer back to a reader or interrogator. Theinformation transfer can be accomplished by modulation of the tagimpedance, for example. Alternatively, other common modulation schemes,such as amplitude shift keying and/or frequency shift keying may also beused.

An Exemplary Wireless System

The present invention further relates to a wireless system including atleast two wireless devices, each wireless device comprising an antennaconfigured to receive a power transmission and broadcast a repetitivemessage, a power converting element coupled to the antenna, and acircuit configured to generate the repetitive message, where the circuitincludes at least one thin film transistor (TFT) configured to introducea random delay in the repetitive message. In some embodiments, thepresent system may be an RFID system, and the wireless devices may beRFID tags. In other embodiments, the repetitive message of the presentsystem is an identification message.

An antenna on a wireless device (e.g., an RFID tag) may be implementedusing a resonant LC circuit for use at 13.56 MHz, for example.Alternatively, the antenna may be implemented using a dipole or similarsuch antenna for 900 MHz or 2.4 GHz operation. However, present wirelesssystems and devices may employ antennas that operate in the RF, HF, VHF,and UHF regimes (e.g. 100-150 KHz, 5-15 MHz, 800-1000 MHz, and 2.4-2.5GHz). Such devices are described in further detail in U.S. patentapplication Ser. Nos. 11/452,108 and 12/467,121 filed Jun. 12, 2006 andMay 15, 2009, the relevant portions of each of which are incorporatedherein by reference. Generally, the antenna may be used to provide powerfor operation of the tag circuitry and to provide information from thetag to a tag reader or interrogator (e.g., a repetitive identificationmessage).

A power converting element on the wireless device may be configured toextract power from an RF transmission (e.g., from an RFID readerstation) by rectifying an RF signal received by the antenna and storingthe resultant charge in a storage capacitor. Thus, when a wirelessdevice enters a region of space with sufficient electromagneticradiation being transmitted from a nearby reader, the storage capacitorbegins to charge-up, and a voltage across the capacitor increasesaccordingly. When the voltage reaches a sufficient value, an “enable”signal can be generated, and this enable signal can be used to initiatethe operation of the wireless device circuit elements and/or circuit(e.g., by powering up and initiating the respective functions of thecircuit elements on an RFID tag). RFID tags suitable for use in thepresent system are described in U.S. patent application Ser. No.11/452,108, filed Jun. 12, 2006, U.S. patent application Ser. No.11/544,366, filed Oct. 6, 2006, U.S. patent application Ser. No.11/203,563, filed Aug. 11, 2005, and U.S. patent application Ser. No.12/249,707, filed Oct. 10, 2008, the relevant portions of each of whichare incorporated herein by reference.

The wireless device may include circuit elements and/or circuits aspreviously described herein configured to receive an initiation signal,and generate a repetitive message in response thereto. For example, thewireless device may include a circuit comprising at least one thin filmtransistor (TFT) configured to introduce a random delay in the broadcastof a repetitive message. In some embodiments, the circuit configured togenerate the repetitive message includes a monostable oscillatorconfigured to generate an oscillating signal having a periodcorresponding to a value of a characteristic electrical parameter of theTFT such as leakage current, on-current, and/or threshold voltage value,where the value of the characteristic electrical parameter fallsrandomly within a predetermined range as previously described herein.

Each wireless device may have a random delay in the broadcast of itsrespective message. Thus, when two (or more) wireless devices in thepresent wireless system are energized, each wireless device antennareceives a power transmission, powers up the wireless device via a powerconverting element coupled to the antenna, and broadcasts a repetitivemessage as preciously described herein. Since each wireless device will(almost certainly) have a slightly different delay in broadcasting themessage back to the reader, collisions between messages from two (ormore) energized wireless devices in a read field may be avoided.Accordingly, the present wireless system provides a mechanism forintroducing random delay in the broadcast intervals of individualwireless devices (e.g., RFID tags) by utilizing random variations in amanufacturing process from producing such wireless devices.

An Exemplary Method of Forming a Collision Tolerant Wireless System

The present invention further relates to method of forming a collisiontolerant wireless system, including determining a target variation rangein the broadcast delay of a plurality of wireless devices havingsubstantially the same architecture, the target range being configuredto reduce a broadcast collision frequency between the wireless deviceswhen the wireless devices broadcast repetitive messages in the same readfield; determining a random variation range in the broadcast delay ofthe wireless devices, comparing the target variation range and therandom variation range; and if the random variation range is at leastequal to (e.g., within) the target variation range, making the wirelessdevices having the random variation range in their broadcast delay. Insome embodiments of the present method, the wireless system is an RFIDsystem, and the wireless devices are RFID tags. In other embodiments,the repetitive message of the present system is an identificationmessage.

Generally, the target range is configured to reduce a broadcastcollision frequency between the wireless devices when the wirelessdevices broadcast repetitive messages in the same read field. The targetrange for the delay in broadcast of repetitive messages is generallyselected such that the minimum span of the target range is broad enough(e.g., τ±m %, where τ is the delay time of the circuit) to ensure thatthe system can accommodate the desired or anticipated number of deviceswhich may be broadcasting simultaneously in a read field withoutcollisions between broadcasts. The maximum span of the target range(e.g., τ±n %, where n>m) may be the maximum range of delay that iscompatible with the operating range of the reader system (e.g.,broadcasts occur at frequencies high enough to be occur at least oncewhile passing through a read field). Thus, a suitable target range mayvary somewhat and can be determined based on the particular parametersrequired for a given application (e.g., the number of devices that maybe transmitting at any given time, and the maximum delay for operabilityof the system).

Determining a target variation range in the broadcast delay of aplurality of wireless devices having substantially the same architecturemay include determining how broad a range of broadcast delays isnecessary for a given or specific application. For example, thedetermination can be made with reference to the number of potentialdevices involved in simultaneous transmission, the frequency oftransmission, the length of the broadcast message, and/or the timelinesand/or robustness specification(s)/requirements of the transmission.However, it is to be appreciated that other considerations arecontemplated in accordance with embodiments of the present invention.

FIG. 7 is a flowchart of an exemplary method for forming a collisiontolerant RFID system. At step 702, a random variation range in thebroadcast delay of the RFID tags is determined. Defining the broadcastdelay range may comprise determining whether a specific architecture,TFT structure, design and/or processing/fabrication method will providea random transmission delay variation as described above. For example,determining the random variation range in the broadcast delay of a lotor production run of RFID tags may comprise testing a representativesample of the RFID tags, and measuring the transmission delay associatedwith each tag, thereby determining the range of delay times associatedwith the tags. Variability in the transmission delay of the RFID tagsmay be implemented in the present approach by exploiting the variationsin the electrical characteristics such as leakage current, on-currentand/or threshold voltage of individual TFTs as described herein. Suchvariability, as described above, is associated with the variability of aprocess or method for manufacturing components of RFID tags. Inaddition, variability in transmission delay can be specificallyintroduced, for example, by implementing individual delay circuit blockswith customizable variability, data driven delay logic, and othersimilar approaches and/or methods.

The target variation range and the measured random variation range ofthe manufactured tags may then be compared. At 703, if the randomvariation range is at least equal to the target variation range, thenthe method concludes with making the RFID tags having the measuredrandom variation range in their broadcast delay. However, when therandom variation range is less than the target variation range, then at704, the present method may include determining whether the randomvariation range can be increased by modifying the manufacturing processby which the RFID tags are manufactured. In some embodiments, modifyingthe manufacturing process comprises changing one or more static designparameters, such as the dimensions and/or composition of one or morecomponents (e.g., the length, width, thickness, topology, dopant and/ordopant level of a channel material in a TFT) of the RFID tag, or one ormore manufacturing parameters (e.g., processing speed, heating and/orcuring temperature and/or time, etc.), or a combination of manufacturingand static design parameters to effect a change in the randomvariability range of the broadcast delay of the RFID tags. For example,modifying the manufacturing process may include making minor changes inthe channel lengths and/or widths of TFTs.

After changing one or more of the manufacturing and/or designparameters, the random variation range of tags produced by such amodified manufacturing process may be measured and compared to a targetvariation range. At 705, when the measured random variation range isincreased by the modification to the manufacturing process, RFID tagsmay be made using the modified manufacturing process. However, ifsufficient delay variation cannot be achieved via changes to themanufacturing process, at 706, a targeted delay may be implemented usingadditional circuit logic (e.g., comprising one or more logic gate(s) asdescribed herein).

Thus, the present method for manufacturing a collision tolerant RFIDsystem enables an integrated design and manufacturing process forwireless devices and systems. Such wireless devices and systems maysimplify and streamline the production of wireless systems for specificapplications (e.g., RFID applications) requiring specific technicalspecifications such as, for example, random variability in the broadcastdelay of tags to be used in the RFID system. Furthermore, the presentmethod may be applied to any system employing multiple wirelessdevices/tags where collision avoidance is desired and device priority isnot important.

CONCLUSION/SUMMARY

Thus, the present invention provides circuits configured to generate arandom delay in wireless communications systems, circuits configured tobroadcast repetitive messages in wireless systems, wirelesscommunication systems, collision tolerant wireless systems, and methodsfor forming such circuits, devices and/or systems. The present inventionadvantageously provides relatively low cost delay generating circuitrybased on TFT technology in wireless electronics applications,particularly in RFID applications. Such novel, technically simplified,low cost TFT-based delay generating circuitry enables novel wirelesscommunications circuits, systems, and method for producing such systems.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the Claims appended hereto and theirequivalents.

What is claimed is:
 1. A circuit configured to generate a delaycomprising: a) a delay element having an input terminal and an outputterminal, the delay element comprising one or more inverters; b) acapacitor having a first terminal receiving an input and a secondterminal coupled to said input of said delay element; and c) a thin-filmfield-effect transistor (TFT) having a first source/drain terminalreceiving a DC source or supply voltage, a second source/drain terminalreceiving said input of said delay element, and a gate electricallyconnected to said second source/drain terminal, said TFT beingconfigured to provide a current and/or voltage to said capacitor,wherein said current and/or voltage has a value that falls randomlywithin a predetermined range.
 2. The circuit of claim 1, wherein saidTFT comprises a material providing said predetermined range of currentand/or voltage.
 3. The circuit of claim 2, wherein said materialcomprises amorphous silicon or polysilicon doped with an amount ofdopant sufficient to control a threshold voltage of said TFT and providesaid predetermined current and/or voltage range.
 4. The circuit of claim1, wherein said TFT has a geometry providing said predetermined range.5. The circuit of claim 1, wherein said delay element comprises aplurality of inverting logic gates.
 6. The circuit of claim 1, whereinsaid one or more inverters comprises an inverter having (i) an inputcoupled to the capacitor and to an output of the TFT, and (ii) an outputcomprising a timing signal.
 7. A monostable oscillator, comprising thecircuit of claim 1, and a feedback path from the output terminal of thedelay element to an input of the circuit.
 8. The monostable oscillatorof claim 7, further comprising a feedback path from the output terminalof the delay element to an input of the circuit.
 9. The monostableoscillator of claim 7, wherein said predetermined range corresponds toan operating current and/or voltage range of a TFT.
 10. The monostableoscillator of claim 7, wherein said value of said current and/or voltageis related to one or more variations in a manufacturing process of saidTFT.
 11. The monostable oscillator of claim 7, wherein said TFTcomprises a channel material comprising an amorphous semiconductingmaterial, a crystalline semiconducting material, or combinationsthereof.
 12. The monostable oscillator of claim 7, wherein said feedbackpath comprises inverter logic having an input coupled to a firstelectrode of the capacitor and an output comprising a timing signal. 13.A circuit configured to broadcast a repetitive message in a wirelesssystem, comprising: a) an antenna configured to receive a powertransmission and broadcast a repetitive message; b) a power-up circuitproviding an initiation signal; c) the monostable oscillator of claim 7,configured to provide a repeating timing signal; d) a memory elementproviding said message; and e) an output circuit configured to broadcastsaid message in response to said timing signal.
 14. The circuit of claim13, further comprising: a) one or more shift register(s); and b) a clockgenerator configured to provide a clock signal to said one or more shiftregisters.
 15. The circuit of claim 13, wherein a period of saidrepeating timing signal corresponds to an RC time constant of said TFTand said capacitor.
 16. The circuit of claim 13, wherein said repetitivemessage comprises a repetitive identification message.
 17. A wirelesssystem comprising at least two wireless devices, each wireless devicecomprising: a) an antenna configured to receive a power transmission andbroadcast a repetitive identification message; b) a power convertingelement coupled to said antenna; and c) the circuit of claim 13,configured to generate said repetitive identification message.
 18. Thesystem of claim 17, wherein said repeating timing signal has a periodcorresponding to a value of a characteristic electrical parameter ofsaid TFT.
 19. The system of claim 18, wherein said characteristicelectrical parameter comprises a leakage current, an on-current, or athreshold voltage.
 20. The system of claim 17, wherein said value ofsaid current and/or voltage is related to one or more variations in amanufacturing process of said TFT.
 21. The system of claim 17, whereinsaid wireless system comprises an RFID system.
 22. The system of claim17, wherein said wireless devices comprise RFID tags.
 23. A method offorming a collision tolerant wireless system, comprising: a) determininga target variation range in a broadcast delay of the wireless devices ofthe wireless system of claim 17, the target variation range beingconfigured to reduce a broadcast collision frequency between saidwireless devices when said wireless devices broadcast repetitivemessages in the same read field; b) determining a random variation rangein the broadcast delay of said wireless devices; c) comparing saidtarget variation range and said random variation range; and d) if saidrandom variation range is at least equal to said target variation range,making said wireless devices having said random variation range in theirbroadcast delay.
 24. The method of claim 23, further comprising: a)determining whether said random variation range can be increased bymodifying the manufacturing process when said random variation range isless than said target variation range; b) when said random variationrange can be increased, modifying said manufacturing process; and c)making said wireless devices using the modified manufacturing process.25. The method of claim 24, wherein modifying said manufacturing processcomprises changing at least one static design parameter, implementing atleast one manufacturing change, or a combination thereof.
 26. The methodof claim 24, wherein said manufacturing process comprises forming atleast one part of a thin film transistor (TFT).
 27. The method of claim23, wherein said wireless system comprises an RFID system.
 28. Themethod of claim 23, wherein said wireless devices comprise RFID tags.29. The method of claim 23, wherein said wireless devices havesubstantially the same architecture.